1. Field of the Invention
This invention relates generally to a portable refrigerating unit for utilization with freeze drying equipment, and more specifically, this invention relates to a condenser coil arrangement for replacing the solvent/dry-ice coolers of the prior art.
2. Description of the Prior Art
While freeze drying is often thought of in connection with freeze dried coffee or dried blood plasma, there are myriad other uses for the freeze drying approach, especially in connection with laboratory testing and preservation. Basically, freeze drying involves lowering the temperature of a liquid sample or product until a completely solid state is reached (i.e., the product is frozen). This sample is then maintained in an area of very low absolute pressure or high vacuum and subjected to a controlled heat input. For many freeze drying applications, the ambient temperature of a room may suffice for the heat input, while for other applications a variable and much more accurately controlled heat input is required. Application of the heat to the product at a controlled rate results in the water content of the frozen sample being sublimated (i.e., converted directly from a solid to a gas without passing through the liquid state). The gaseous water vapor is then refrozen on the refrigerating unit for removal from the system, protecting the vacuum pump oil from water vapor contamination and resultant loss of pumping ability.
Various types of freeze drying units have been utilized in the past. In one type of prior art device, a large cabinet assembly incorporates a compressor, a vacuum pump and all the other equipment needed for the freeze drying operation. In one such device manufactured by The Virtis Company, a division of the assignee of this application, a stainless steel mechanically refrigerated condenser coil extends from the top of the cabinet into the drying chamber of the freeze drying apparatus. The condenser coil and the drum or manifold are affixed to the unit and form an integral portion thereof. While these devices have many uses, for many other applications they are too expensive, large, bulky, unwieldy and inflexible to meet a given need.
Accordingly, smaller, relatively inexpensive portable type freeze drying units have also been utilized in the past. In this type of device, the refrigerating unit is provided by a solvent bath cooled by dry-ice. While this type of device provides the desired portability for various uses, such as laboratory bench work, it does have the disadvantage that the dry-ice must be replenished at periodic intervals, often after normal working hours when the laboratory is unattended. In addition, the necessity of the solvent bath creates additional handling and flammability problems.
Some attempts have been made to improve upon the dry-ice portable freeze dryers by utilizing mechanically refrigerated condenser coils that are immersed in the solvent bath. These devices utilize a portable compressor that conveys refrigerant to the condenser coil through a flexible tube. However, these appproaches do not eliminate the necessity of handling the solvent bath. Further, this type of approach involves the additional problem of getting the condenser coil into the solvent bath and adds the disadvantage that it cools the solvent very slowly, unlike the dry-ice which it replaces.